Gas turbine engines may generate electricity in addition to the motive force they provide. Generators to convert kinetic energy into electricity occupy space and add weight to the aircraft. However, adding weight and consuming onboard space in an aircraft may reduce the overall efficiency of the aircraft.
Generators may typically be driven by the high-pressure spool. The angular velocity of the high-pressure spool may be in a narrow range at high revolutions per minute (“RPM”). For example, a shaft from the high-pressure section of a gas turbine engine may rotate at a speed range with the lowest speed within a ratio of 2 to 1 to the highest speeds (e.g., 10,000 to 20,000 RPM). The narrow range may enable a generator to operate at a desired voltage at the minimum RPM without overloading at the maximum RPM. The high rotational speed of the high-pressure spool may also spin a generator at high RPM and thus enable smaller generators to be employed. However, the high pressure section may not provide enough energy to satisfy the electric demand of the aircraft as well as the desired motive force.
The low-pressure section of a gas turbine engine usually operates at lower speeds with a broader range of speeds than the high-pressure section. For example, a shaft from a low pressure section may rotate at a higher ratio of speed ranges (e.g., 5 to 1). Low input speeds and high ranges of speeds may present difficulties when designing a generator that will not overload at the high RPMs. Lower input speeds may thus translate to higher weight and larger volume generators. Larger generators in an aircraft may result in reduced aircraft efficiency.